I. Field of the Invention
The present invention relates to data transmissions. More particularly, the present invention relates to a bi-directional vector rotator capable of rotating the phase of a signal in clockwise and counter clockwise directions.
II. Description of the Related Art
In a typical digital communications system, data is processed at a transmitter device, modulated using a particular modulation scheme, filtered, amplified, and transmitted to one or more receiver devices. The modulation scheme can be M-ary phase shift keying (e.g., BPSK, QPSK, OQPSK, and so on), M-ary quadrature amplitude modulation (i.e., M-QAM), or some other scheme. At a receiver device, the modulated signal is received, conditioned, demodulated, and processed to recover the transmitted data.
Depending on the particular modulation scheme used at the transmitter device, the demodulation typically includes converting the received modulated signal into inphase (I) and quadrature (Q) signals that are further processed. The quadrature downconversion can be performed using analog or digital circuitry. In a typical analog implementation, the modulated signal is multiplied with inphase and quadrature carrier signals to generate the inphase and quadrature signals, respectively. And in a typical digital implementation, the modulated signal is digitized at a high frequency and digitally multiplied with the inphase and quadrature carrier signals to generate inphase and quadrature samples.
For some receiver designs, a vector rotator is employed in conjunction with a carrier tracking loop to lock the phase of a complex carrier signal to that of the received modulated signal. The modulated signal is initially downconverted to near DC by an intermediate frequency (IF) carrier signal that is free running and not locked to the modulated signal. The vector rotator then performs a complex multiply of the downconverted signal with the complex carrier signal to generate a phase rotated output signal. The complex carrier signal is typically a lower frequency signal that is synthesized based on a clock signal. The phase of the complex carrier signal is adjusted by the carrier tracking loop and locked to that of the modulated signal (i.e., the phase of the output signal has 0xc2x0 phase offset when the carrier tracking loop is locked). When the carrier tracking loop is not locked, phase rotation in the output signal is used to adjust the phase of the complex carrier signal such that the phase rotation is reduced and removed.
A conventional vector rotator typically performs phase rotation in one direction (i.e., clockwise or counter clockwise) and is able to translate a modulated signal in one direction (i.e., either down to a lower frequency or up to a higher frequency). For the receiver designs described above, the initial frequency downconversion typically places the modulated signal at a particular positive frequency such that the vector rotator can perform the frequency translation or quadrature downconversion (in one direction) from the positive frequency down to DC.
In certain applications, phase rotation in both directions is needed and a second vector rotator is provided. The additional vector rotator typically equates to additional hardware, which increases the complexity and costs of the device. Moreover, the carrier signal generator typically needs to be modified to provide a second phase for the second vector rotator, thereby further increasing the complexity of the design.
Thus, a bi-directional vector rotator capable of performing phase rotation in both clockwise and counter clockwise directions is advantageous for some applications and is highly desirable.
The present invention provides a bi-directional vector rotator that can be used to generate outputs having phases that are rotated in clockwise and counter clockwise directions relative to that of the input signal. The bi-directional vector rotator includes a product term generator that receives a complex input and a complex carrier signal and generates product terms. Combiners then selectively combine the product terms to generate the outputs. By sharing the same product term generator for both clockwise and counter clockwise phase rotations, the bi-directional vector rotator can be implemented using less circuitry than that for a conventional design employing two uni-directional vector rotators. Moreover, only one complex carrier signal is needed by the bi-directional vector rotator instead of two for the conventional design. Further simplification in the design of the bi-directional vector rotator can be achieved by selecting the proper sampling rate for the complex input, as described below. The bi-directional vector rotator can be advantageously used in a receiver device, and is especially efficient in demodulating a multi-carrier signal having multiple (e.g., three) modulated signals.
An embodiment of the invention provides a bi-directional vector rotator that includes a product term generator coupled to first and second sets of combiners. The product term generator receives and multiplies a complex input with a complex carrier signal to generate a set of product terms. Each product term is representative of a particular combination of a real or imaginary component of the complex input and a real or imaginary component of the complex carrier signal. The first set of combiners selectively combines the product terms to generate a first complex output having a phase that is rotated in a first direction (e.g., clockwise) relative to that of the complex input. The second set of combiners selectively combines the product terms to generate a second complex output having a phase that is rotated in a second direction (e.g., counter clockwise) relative to that of the complex input.
The product term generator can be implemented in various manners. For example, the product term generator can be implemented with a set of multipliers, with each multiplier multiplying the real or imaginary component of the complex input with the real or imaginary component of the complex carrier signal to provide a respective product term. The product term generator can also be implemented with one or more look-up tables, combinatory logic, and others (e.g., hardware, software, or a combination thereof).
In a specific design, the complex input comprises digitized inphase and quadrature samples, IIN and QIN, the complex carrier signal comprises inphase and quadrature carrier signals, cos(xcex8) and sin(xcex8), and the product terms comprises IIN cos xcex8, QIN sin xcex8, QIN cos xcex8, and IIN sin xcex8. The first set of combiners adds the product terms IIN cos xcex8 and QIN sin xcex8 to generate inphase samples, IROTcw, and subtracts the product term IIN sin xcex8 from the product term QIN cos xcex8 to generate quadrature samples, QROTcw. Similarly, the second set of combiners subtracts the product term QIN sin xcex8 from the product term IIN cos xcex8 to generate inphase samples, IROTccw, and adds the product terms IIN sin xcex8 and QIN cos xcex8 to generate quadrature samples, QROTccw. The resolution of the first and second complex outputs can be maintained similar to that of the complex input (e.g., 4 bits, or some other resolution).
Another embodiment of the invention provides a method for generating signals having phases that are rotated in clockwise and counter clockwise directions. In accordance with the method, a complex input and a complex carrier signal, each having real and imaginary components, are received and used to generate a set of product terms. Each product term is representative of a particular combination of a real or imaginary component of the complex input with a real or imaginary component of the complex carrier signal. The product terms are then selectively combined to generate a first complex output having a phase that is rotated in the clockwise direction relative to that of the complex input. The product terms are further selectively combined to generate a second complex output having a phase that is rotated in the counter clockwise direction relative to that of the complex input. The first and second complex outputs may be truncated or rounded to a resolution similar to that of the complex input. Again, the product terms can be generated by a set of multipliers, one or more look-up tables, combinatory logic, or others.
Another embodiment of the invention provides a receiver for use in a wireless communications system. The receiver includes a front-end unit coupled to a bi-directional vector rotator. The front-end unit receives, conditions, and digitizes a received signal to provide a complex input signal. The bi-directional vector rotator multiplies the complex input signal with a first complex carrier signal to provide first and second complex outputs. The first complex output has a phase that is rotated in a first direction (e.g., clockwise) relative to that of the complex input signal, and the second complex output has a phase that is rotated in a second direction (e.g., counter clockwise) relative to that of the complex input signal.
In a specific application, the received signal is a multi-carrier signal conforming to a CDMA-2000 standard (identified below) and including three modulated signals centered at first, second, and third frequencies. The first complex output then corresponds to the modulated signal centered at the first frequency, and the second complex output corresponds to the modulated signal centered at the third frequency. For ease of implementing the bi-directional vector rotator, if the first, second, and third frequencies are separate by a particular sub-carrier frequency, fsc, the complex input signal can comprise complex samples having a sample rate, fs, that is a multiple of four times the sub-carrier frequency (i.e., fs=4fsc, 8fsc, and so on).
The receiver typically further includes a first tracking loop coupled to a numerically controlled oscillator (NCO), and a second tracking loop coupled to a voltage controlled oscillator (VCO). The first tracking loop receives the first or second complex output, or both, and provides a first control signal. The NCO receives the first control signal and generates the first complex carrier signal in response. The second tracking loop receives the complex input signal and provides a second control signal. The VCO receives the second control signal and generates a second complex carrier signal that is then used by the front-end unit (e.g., for quadrature downconversion) to provide the complex input signal.